This invention relates to an adaptive pulsing system for controlling a motor, which moves a machine or other device accurately to a preselected new position. The invention is useful wherever precise control of position is desired and is particularly useful if "overshoot" correction is a problem. There are many uses for control systems of this type, but one of the primary reasons for development of the present invention relates to problems inherent in devices moved by lead screws. Lead screw driven devices have a "backlash" problem if the position feedback information is derived from measurement of the lead screw rotation rather than measurement of the position of the element driven by the lead screw. If the movement of the device in the initial direction leads to an "overshoot", the corrective motion in the reverse direction has an unacceptable hysteresis, or margin of error, because of the backlash in the lead screw driving mechanism. Therefore, the aim in such a mechanism is to cause the device to "settle into" its final position by means of movement in one direction only, and to always move into its final position from the same direction.
While the problem of hysteresis, or position inaccuracy due to "hunting", can be at least partially avoided by direct measurement of the location of the element whose position is being changed, this is often highly impractical and excessively expensive. For one thing, the distance measurement must be continuous, and must relate to a specific starting point. In other words, continuous base point to end measurement is required. If large distances are to be covered, as on a large machine table, a highly accurate, direct measurement of the moving element is extremely difficult. For this reason, it is very desirable to use lead screw measurement to determine position.
The problem of inaccuracy due to hunting also arises wherever there is "slack", or elasticity, in the driving system, if it is desired to measure position from the position of the driving mechanism. Such elasticity or slack exists in belt-driven systems, geared drives, fluid couplings, and the like.
In Sweeney et al. Application Ser. No. 62,416, filed July 31, 1979, titled "Digital Motor Control for Positioning System", a control apparatus and method are disclosed which provide a significant advance in solving the problems discussed above. The system of that application uses a shaft encoder to provide distance-representing signals which cause a driving motor to be turned on and off in accordance with incremental distance demands. In the preferred version shown in that application, the distance demands are reduced incrementally during the final stage of the positioning movement, the size of subsequent distance increments being automatically determined as a function of the remaining distance to destination. Because of the digital nature of the control system, the increments used are generally either one-half, one-fourth or one-eighth of the remaining distance.
The position control system of Ser. No. 62,416 has been highly successful, but experience has demonstrated that even greater positioning accuracy is desirable. If the size of the decreasing increment is too large, the system may "overshoot" the destination in a certain percentage of runs. On the other hand, if the size of the decreasing increment is made smaller, in order to avoid overshoot, the position accuracy is diminished. For example, a decreasing increment of one-half of the remaining distance may cause overshoot due to the momentum of the machine. If the size of the decreasing increment is reduced to one-fourth of the remaining difference, the tolerance of the final accuracy will be approximately four times the measurement resolution of which the system is capable. In other words, if the system has a potential accuracy of one mil, any remaining distance after the last pulse which is less than four mils will not cause generation of a further (one mil) pulse.
If overshoot does occur in a given application, due to use of too large an approach increment, the result may be oscillation, because the overshoot returns the machine to the approach mode, and it is possible to set up an alternating sequence of overshoot and return movements. In order to avoid any overshoot problem, some installations of the system disclosed in application Ser. No. 62,416 have used a one-eighth approach increment. In general, it can be stated that any system which tends to develop oscillation back and forth across the destination point will have to sacrifice some accuracy in order to solve the oscillation problem.
Where a stepper motor control is used to provide a series of equal, pulse-controlled steps, for the purpose of attaining accuracy and avoiding overshoot, the costs are very high. A relatively large stepper motor is usually necessary in order to overcome friction in the machine; and the stepper motor constitutes an extra, final stage in addition to other means for moving the machine to the beginning of the final stage. Another problem in stepper motor systems is that the larger stepper motors, in particular, have torque requirements which make it very difficult to move them precisely.
In general, the purpose of the present invention is to eliminate the shortcomings of the earlier systems by (a) providing much greater accuracy in reaching the destination, (b) adapting automatically to varying frictional and inertial loading of the driven element, and (c) preventing oscillation due to overshoot. These advantages are applicable to any position control system, and are in no way limited to the system disclosed in Ser. No. 62,416. The present invention will, however, normally be used as the final, precise positioning stage in a system wherein the apparatus disclosed in Ser. No. 62,416, or some other known apparatus, is used to bring the driven element very close to its destination.
Certain other shortcomings of the positioning system disclosed in Ser. No. 62,416 are also dealt with in the present invention. The earlier application showed an AC motor as the pulse-controlled driver for the driven element. Experience has shown that such a system had inherent limitations. One was a limit on the timing accuracy, or precision, obtainable in turning the motor on and off, which made it impossible for such a system to fully utilize the potential accuracy attainable by using the present invention. Another such limitation was the difficulty of varying motor speed over a wide range, because the AC motor speed must be adjusted by varying frequency; and therefore it would not usually be feasible to use a single AC motor to provide both the fast and slow stages of the approach procedure.
Another shortcoming of the system disclosed in Ser. No. 62,416 was the inclusion of a clutch between the slow-speed AC motor and the driven element. The time required for engagement and disengagement of the clutch was far greater than the duration of the pulses utilized in the present invention, thereby rendering the precision of the present system largely inapplicable to the installations which included clutches in the driving structure.